Bolt tightening structure of magnesium alloy member

ABSTRACT

The present invention provides a bolt tightening structure of a magnesium alloy member capable of insulating between magnesium alloy members and steel or other bolts without lowering the bolt shaft power, and preventing occurrence of electrolytic corrosion if contacting with electrolyte such as water. This is a bolt tightening structure of a magnesium alloy member for tightening magnesium alloy members with each other or a magnesium alloy member and a heterogeneous material by means of a bolt. Cationic electrodeposition coating having a film thickness of 15 μm or more is provided at least on the surface of the magnesium alloy member contacting with a bolt head, a powder coating having a film thickness of 40 to 150 μm is provided on surface of the cationic electrodeposition coating, zinc-nickel plating and then cosmer treatment are carried out on the bolts, and an alumite-treated aluminum washer is interposed between the bolt head and the magnesium alloy member.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a bolt tightening structure of amagnesium alloy members, and more particularly to a technology forpreventing occurrence of electrolytic corrosion of the magnesium alloymember.

[0002] Recently, in the automobile industry, as the concern aboutenvironmental problems increases, a further improvement of fuel economyhas been demanded. To meet such demand, in the automobile industry, itis required to reduce the weight of the car body, and magnesium alloysare widely used in automotive parts owing to their lightest weight amongpractical metals. More lately, in particular, they are applied inlocations where very high corrosion resistance is required such asexterior and structural parts.

[0003] However, since the magnesium alloy is the basest practical alloy,when it is tightened with a heterogeneous metal such as iron oraluminum, electrolytic corrosion is likely to occur in the presence ofmoisture containing electrolyte. Especially for automobiles usedoutdoors, electrolytic corrosion is extremely promoted by the functionof electrolyte contained in rainwater, seawater, or snow-melting salt,possibly causing breakage or damage of parts.

[0004] Japanese Laid-open Patent No. 5-302614 proposes a technology forpreventing electrolytic corrosion by interposing a paper washer coatedwith resin between a magnesium alloy member and a steel bolt so as toavoid contact between the bolt and the magnesium alloy member. However,as the bolt corrodes, corrosion products flow out onto the magnesiumalloy to cause electrolytic corrosion, and therefore it was impossibleto apply in the position where corrosion resistance was required. It maybe also considered to insulate between a bolt head and a magnesium alloymember by means of a resin washer, but the resin creeps by heat incoating, and the bolt shaft power lowers, and retightening is required,and hence it is not practicable. Moreover, recently, automakers inEurope have developed techniques for interposing an organic materialcontaining inorganic fillers between a magnesium alloy member and abolt. In such techniques, however, after 20 cycles of compositecorrosion test by salt spray, occurrence of electrolytic corrosion wasnoted, and it was far from satisfaction of recent demands.

SUMMARY OF THE INVENTION

[0005] It is hence an object of the invention to provide a bolttightening structure of a magnesium alloy member capable of insulatingbetween a magnesium alloy member and a steel bolt without lowering thebolt shaft power, and preventing occurrence of electrolytic corrosioneven in contact with electrolyte such as water.

[0006] The present invention is a bolt tightening structure of amagnesium alloy member for tightening magnesium alloy members with eachother or a magnesium alloy member and a heterogeneous material by meansof a bolt, wherein a cationic electrodeposition coating having a filmthickness of 15 μm or more is provided at least on the surface of themagnesium alloy member contacting with a bolt head, a powder coatinghaving a film thickness of 40 to 150 μm is provided on the surface ofthe cationic electrodeposition coating, zinc-nickel plating and thencosmer treatment are carried out on the bolt, and an alumite-treatedaluminum washer is interposed between the bolt head and the magnesiumalloy member.

[0007] As investigated by the present inventors, without cationicelectrodeposition coating, even if a powder coating is applied in asufficient thickness, the adhesion of the powder coating is poor, andlocal surface corrosion or electrolytic corrosion is likely to occur. Incontrast, when a cationic electrodeposition coating is applied, theadhesion of the coating is sufficient, and such defect does not occur.However, when the film thickness of the cationic electrodepositioncoating is less than 15 μm corrosion resistance is extremely lowered,and hence the film thickness is preferably 15 μm or more. With thecationic electrodeposition coating only, the cationic electrodepositioncoating film may be scratched by torque when tightening a bolt, andelectrolytic corrosion is likely to grow from flaws. It is thereforeessential to apply powder coating after cationic electrodepositioncoating as proposed in the present invention. If the film thickness ofthe powder coating is less than 40 μm the coating appearance isextremely deteriorated and the film thickness is locally reduced tocause electrolytic corrosion easily. In contrast, if the film thicknessis over 150 μm, the shaft power is lowered when tightening the bolt.

[0008] On the other hand, the bolt is treated by zinc-nickel plating andthen cosmer process. Generally, for a bolt used in tightening of amagnesium alloy member, tin-zinc plating is preferable because thepotential difference from magnesium is small. However, tin-zinc platingis not so superior in corrosion resistance, and the bolt corrode easily.When the bolt corrode, corrosion products flow to cause electrolyticcorrosion between the corrosion products and magnesium alloy. The bolttreated by zinc plating and cosmer process is superior in corrosionresistance, and there is no problem in tin-zinc plating, and such boltis easily available commercially.

[0009] As far as a bolt is made of steel, when used alone, potentialdifference from magnesium alloy cannot be eliminated completely. Whentightening the bolt, moreover, due caution is needed not to scratch thecoat surface by the bolt head. From such viewpoint, it is required touse an insulating washer in order to protect and insulate the magnesiumalloy member. In this case, considering the insulation, corrosionresistance, and bolt shaft power, an alumite-treated washer are ideal.Alternatively, an iron or aluminum alloy washer treated by providing acationic electrodeposition coating having a film thickness of 15 μm ormore can also be used. In this case, if the film thickness of thecationic electrodeposition coating is less than 15 μm, current maylocally pass, and depending on the bolt tightening torque or number oftimes of repetition of tightening, there is a slightly possibility ofoccurring electrolytic corrosion on the magnesium alloy member.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows photographs of test pieces after salt spray testaccording to Examples 1 to 5 of the present invention.

[0011]FIG. 2 shows photographs of test pieces after salt spray testaccording to Examples 6 to 9 of the present invention.

[0012]FIG. 3 shows photographs of test pieces after salt spray testaccording to Comparative Examples 1 to 6.

[0013]FIG. 4 shows photographs of test pieces after salt spray testaccording to Comparative Examples 7 to 12.

[0014]FIG. 5 shows photographs of a wide range of test pieces after saltspray test according to Examples 1 and 4, and Comparative Example 8.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] Preferred embodiments of the present invention are describedbelow.

[0016] 1. Magnesium Alloy Member

[0017] A magnesium alloy member preferably contains Al in an amount of5% by weight or more. If the content of Al is less than 5% by weight,corrosion resistance of the magnesium alloy member is lowered, andcorrosion resistance of coated surface decreases, and it is not suitedto bolt tightening. The surface of the magnesium alloy member ispreferred to be treated by any one of chromic acid chrome treatment(chromate treatment), chromium phosphate treatment, and manganesephosphate treatment. Cr or Mn is contained in the surface of themagnesium alloy member by such treatment, and by the oxidation andreduction reaction of these elements (change of valence), oxidationreaction of magnesium is suppressed, so that electrolytic corrosion maybe prevented effectively.

[0018] Powder coating applied on the surface of a cationicelectrodeposition coating preferably contains epoxy resin and polyesterresin as a principal component. Considering outdoor use, these resinsare highest in light fastness and superior in corrosion resistance.

[0019] 2. Bolt

[0020] After zinc-nickel plating and before cosmer treatment, a bolt ispreferred to be treated by chromate treatment. As a result, corrosionresistance of the bolt is further enhanced, and electrolytic corrosionof the magnesium alloy member by corrosion products can be effectivelyprevented. Cosmer treatment is generally a process for coating boltsurface with ethylene-acrylic acid copolymer resin dispersion addedwater dispersing silica, silane coupling agent, and water dispersingchromium compound.

[0021] 3. Washer

[0022] In order to prevent tearing of an alumite layer and assureinsulation when tightening bolt, the thickness thereof is preferred tobe 10 μm or more, and more preferably 30 μm or more. Further, by coatingadhesive or thermoplastic resin sealer on the bolt tightening area,infiltration of electrolyte is prevented and resistance to electrolyticcorrosion is enhanced.

[0023] 4. Tightening Part

[0024] In the present invention, material of part to be tightened withthe magnesium alloy member is assumed mainly to be iron or aluminumalloy. Of such tightening part, at least the surface contacting with themagnesium alloy member is preferably treated by cationicelectrodeposition coating. The film thickness thereof is preferred to be15 μm or more. The reason is the same when applying cationicelectrodeposition on the magnesium alloy member. When an iron part istreated by zinc plating or zinc-nickel plating prior to such coating,the corrosion resistance may be further enhanced.

EXAMPLES

[0025] Examples

[0026] In the following, the present invention is further explained indetail by referring to Examples.

[0027] 1. Preparation of Test Piece

[0028] In a plate material of magnesium alloy (AM60B or AZ91D) of 70 mmin width, 150 mm in height and 3 mm in thickness, 6 holes of 9 mm indiameter were formed, and burrs were removed sufficiently. Suchmagnesium alloy plate was prepared as a test piece of Examples 1 to 5,and was pretreated and coated as shown in Table 1. In chromatetreatment, chromic acid chrome treatment solution (trade name: NH35,produced by Norsk Hydro Co., Ltd.) was used and chrome loading was 0.5g/m². In manganese phosphate treatment, Mag Plus treating solution(trade name), produced by Ebina Electric Co., Ltd., was used. Incationic electrodeposition coating, Power Top V50 (trade name) producedby Nippon Paint Co., Ltd., was used, and the film thickness was 25 μm or15 μm. In powder coating, an epoxy/polyester paint (trade name: PowduxP60, produced by Nippon Paint Co., Ltd.) was used, and the filmthickness thereof was 100 μm or 40 μm.

[0029] As a bolt, a steel flanged bolt of 8 mm in diameter, beingtreated by zinc-nickel plating, colored chromate treatment, and cosmertreatment (Strength 7T, produced by Ohashi Technica Co., Ltd.), wasused. As a washer, a light alumite washer made of aluminum, beingtreated by sulfuric acid treatment and sealed by nickel acetate method(JIS A5182, produced by Ohashi Technica co., Ltd.), was used. Thethickness of an alumite layer of the washer was set at 10 μm or 30 μm.

[0030] 2. Bolt Tightening

[0031] As a tightening part, an iron plate in which screw holes suitedto the bolt diameter were formed, zinc phosphate treatment was carriedout and a cationic electrodeposition coating having a film thickness of15 μm was provided, was used. A magnesium alloy test piece was put onthe iron plate, and tightened by a bolt and a washer. The bolt wastightened by an air impact wrench to specified torque. Lowering of shaftpower of the bolt after tightening was investigated. With respect to thebolt shaft power test, after tightening the bolt to specified torque, itwas held at 100° C. for 60 minutes, and then the shaft power thereof wasmeasured. In Table 1, the following criteria were used: cases where themeasured value was lower than the initial shaft power by more than 20%:X; and cases where it was within 20%: ο. When lowering of the shaftpower is within 20%, retightening process can be omitted by setting theinitial shaft power slightly higher.

[0032] 3. Salt Spray Test

[0033] The test pieces after bolt tightening were evaluated by saltspray test to investigate about swelling of the coat film and occurrenceof electrolytic corrosion near a bolt tightening area in the magnesiumalloy test piece. The results are shown in Table 1. In the salt spraytest, 5% NaCl solution was sprayed to the test piece at a flow rate of1.5 ml/min for 1200 hours. “Swelling of coat film” refers as a state inwhich the coat film near a bolt tightening area of a magnesium alloymember changes in appearance due to corrosion of the lower side thereof.“Electrolytic corrosion” refers as a state in which a large-scalepitting corrosion accompanied by peeling of a coat film occurs near abolt tightening area. Further, it was investigated whether corrosionoccurred at the edge of the test piece due to flow-out of corrosionproducts produced by corrosion of the bolt, and the results are alsoshown in Table 1. TABLE 1 Ordinary torque Exam- Exam- Exam- Exam- Exam-tightening ple 1 ple 2 ple 3 ple 4 ple 5 Magnesium alloy member MaterialAM60B AM60B AM60B AM60B AZ91D Chromate treatment Treated Treated TreatedTreated Manganese phosphate Treated treatment Cationic electrode-Treated Treated Treated Treated position coating (25 μm) Cationicelectrode- Treated position coating (15 μm) Powder coating (100 μm)Treated Treated Treated Treated Powder coating (40 μm) Treated WasherAlumite treatment Treated Treated Treated Treated (10 μm) Alumitetreatment Treated (30 μm) Bolt ZnNi plating/ Treated Treated TreatedTreated Treated Cosmer treatment Salt spray test result Swelling of coatfilm None None None None None in bolt tightening area Electrolyticcorrosion of None None None None None material in bolt tightening areaCorrosion at material None None None None None edge Tighteningperformance Decrease of bolt shaft None None None None None powerOverall evaluation ∘ ∘ ∘ ∘ ∘

[0034] As shown in Table 1, in Examples 1 to 5, after bolt tightening,the shaft power was not lowered at all. FIG. 1 shows photographs of testpieces after salt spray test according to Examples 1 to 5. As shown inTable 1 and FIG. 1, in the magnesium alloy test pieces, swelling of coatfilm and electrolytic corrosion in the bolt tightening area did notoccur at all, and edge corrosion was not recognized.

[0035] 3. Bolt Over-torque Tightening and Salt Spray Test

[0036] The same magnesium alloy plates as in Examples 1 to 5 wereprepared as test pieces in Examples 6 to 9, and were pretreated andcoated as shown in Table 2. The magnesium alloy test pieces weretightened to iron plates in the same manner as in the above Examples. Atthis time, the bolt was tightened to torque of 1.2 times that of theabove Examples, and this over-torque tightening and loosening wererepeated 10 times. After the same salt spray test, swelling of coat filmnear the bolt tightening area of the magnesium alloy test piece,presence or absence of electrolytic corrosion, and corrosion at the edgeof the test piece were investigated. The results are also shown in Table2. TABLE 2 Over-torque tightening reteated 10 times Example 6 Example 7Example 8 Example 9 Magnesium alloy member Material AM60B AM60B AM60BAM60B Chromate treatment Treated Treated Treated Treated CationicTreated electrodeposition coating (25 μm) Cationic Treated TreatedTreated electrodeposition coating (15 μm) Powder coating (100 μm)Treated Powder coating (40 μm) Treated Treated Treated Washer AlumiteTreated Treated Treated treatment (10 μm) Alumite Treated treatment (30μm) Bolt Treated Treated Treated Treated ZnNi plating/cosmer treatmentSalt spray test result Swelling of coat film None None Present Presentin bolt tightening area Electrolytic corrosion of None None None Nonematerial in bolt tightening area Corrosion at material None None NoneNone edge Overall evaluation ⊚ ⊚ ◯ ◯

[0037]FIG. 2 shows photographs of test pieces after salt spray testaccording to Examples 6 to 9. As shown in Table 2 and FIG. 2, in themagnesium alloy test pieces of Examples 6 and 7, swelling of coat filmin bolt tightening area and electrolytic corrosion did not occur at all,and edge corrosion was not recognized. In contrast, in the magnesiumalloy test pieces of Examples 8 and 9, swelling of coat film in bolttightening area was recognized. It is estimated to be due to effects ofa powder coating having a thickness of 40 μm and an alumite layer ofwasher having a thickness of 10 μm.

[0038] Overall evaluations were carried out by the above results, andthe evaluations are shown in Table 1 and Table 2. In the overallevaluation, the following criteria were used: cases where a problemoccurred in any one of the items in bolt tightening at specified torque:X; cases where no problem occurred at specified torque: ο; and caseswhere no problem occurred at over-torque: ⊚. As a result, the result was⊚ in Examples 6 and 7, and ο in all other Examples.

[0039] Comparative Examples

[0040] The same magnesium alloy plates as in the above Examples wereprepared as test pieces of Comparative Examples 1 to 12. These testpieces were pretreated and coated as shown in Table 3 and Table 4. Usingbolts and washers specified in Table 3 and Table 4, and the same ironplates as in the above Examples, the bolt was tightened to specifiedtorque. Herein, “chelating treatment” shown in Table 3 and Table 4 is akind of pretreatment for coating, and it refers as a treatment forchelating Mg component by treating a magnesium alloy plate with azolehaving a hydrazine structure. Other pretreatments and coating conditionswere the same as in the above Examples. TABLE 3 Com- Com- Com- Com- Com-Com- Ordinary parative parative parative parative parative parativetorque Exam- Exam- Exam- Exam- Exam- Exam- tightening ple 1 ple 2 ple 3ple 4 ple 5 ple 6 Magnesium alloy member Material AM60B AM60B AM60BAM60B AM60B AM60B Chromate Treated Treated Treated Treated Treatedtreatment Chelating Treated treatment Cationic Treated Treated TreatedTreated electrode- position coating (25 μm) Powder coating TreatedTreated Treated Treated Treated (100 μm) Powder coating (200 μm) WasherAlumite Treated Treated Treated Treated Treated treatment (30 μm) IronIron Washer Resin Bolt ZnNi plating/ Treated Treated Treated Treatedcosmer treatment Dacro Treated treatment Chrome plating Treated Zincplating Tin-zinc plating Salt spray test result Swelling of PresentPresent Present Present None None coat film in bolt tightening areaElectrolytic Present Present Present None None None corrosion ofmaterial in bolt tightening area Corrosion at None None None NonePresent Present material edge Tightening performance Decrease of NoneNone None None None None bolt shaft power Overall x x x x x x evaluation

[0041] TABLE 4 Com- Com- Com- Com- Com- Com- Ordinary parative parativeparative parative parative parative torque Exam- Exam- Exam- Exam- Exam-Exam- tightening ple 7 ple 8 ple 9 ple 10 ple 11 ple 12 Magnesium alloymember Material AM60B AM60B AM60B AM60B AM60B AM60B Chromate TreatedTreated Treated Treated Treated Treated treatment Chelating treatmentCationic Treated Treated Treated Treated Treated electrode- positioncoating (25 μm) Powder coating Treated Treated Treated Treated Treated(100 μm) Powder coating Treated (200 μm) Washer Alumite Treated TreatedTreated Treated treatment (30 μm) Iron Resin Resin Washer Bolt ZnNiplating/ Treated Treated Treated cosmer treatment Dacro treatment Chromeplating Zinc plating Treated Tin-zinc Treated Treated plating Salt spraytest result Swelling of None None None None Present Present coat film inbolt tightening area Electrolytic None None None None Present Presentcorrosion of material in bolt tightening area Corrosion at PresentPresent None None None Present material edge Tightening performanceDecrease of None None Present Present None None bolt shaft power Overallx x x x x x evaluation

[0042] As a washer, an iron washer and a resin (plastic) washer wereused, except for the almite washer having an alumite layer of 30 μm. Asa bolt, bolts treated by Dacro treatment, chrome plating, zinc plating,and tin-zinc plating were used, except for the bolts used in the aboveExamples. “Dacro treatment” refers as a process treated by dacrotizedtreating solution (produced by Dacro Shamrock Japan Co., Ltd.).

[0043] With respect to these test pieces of Comparative Examples, aftersalt spray test was carried out in the same condition as that inExamples, swelling of coat film near the bolt tightening area of themagnesium alloy test piece, presence or absence of electrolyticcorrosion, and corrosion at the edge of the test piece wereinvestigated. The results are shown in Table 3 and Table 4, andphotographs of test pieces after salt spray test are shown in FIG. 3 andFIG. 4. In the following, the Comparative Examples will be analyzed.

[0044] In Comparative Example 1 in which powder coating was not appliedon the magnesium alloy plate and in Comparative Examples 2 and 3 inwhich no cationic electrodeposition coating was provided, swelling ofcoat film and electrolytic corrosion occurred near the bolt tighteningarea of the magnesium alloy plate. In Comparative Example 4 using aniron washer, swelling of coat film occurred near the bolt tighteningarea of the magnesium alloy plate. In Comparative Examples 5, 6, 7, and8 using a bolt treated by Dacro treatment, chrome plating, zinc plating,and tin-zinc plating only, respectively, corrosion of the bolt wassignificant, and corrosion products flowed out to corrode at the edge ofthe test piece. FIG. 5 shows photographs of test pieces after salt spraytest of Examples 1, 4 and Comparative Example 8, and a flow-out state ofcorrosion products and corrosion at the edge of the test piece wereobserved in Comparative Example 8.

[0045] In Comparative Example 9, since the film thickness of powdercoating was 200 μm which exceeds the upper limit (150 μm) of the presentinvention, and in Comparative Example 10, since the resin washer wasused, although there was no problem in the coat film, the bolt shaftpowers were lowered. In Comparative Example 11, since no washer wasused, swelling of coat film and electrolytic corrosion occurred near thebolt tightening area of the magnesium alloy plate. In ComparativeExample 12, since no cationic electrodeposition coating was provided anda tin-zinc plated bolt was used, swelling of coat film and electrolyticcorrosion occurred near the bolt tightening area of the magnesium alloyplate, and corrosion at the edge of the test piece due to corrosionproducts of the bolt was also detected.

[0046] Overall evaluations of Comparative Examples 1 to 12 were carriedout by the above results and the results are shown in Table 3 and Table4.

[0047] In all Comparative Examples, since there were problems when thebolt was tightened to specified torque, the overall evaluations were X.

What is claimed is:
 1. A bolt tightening structure of a magnesium alloymember for tightening magnesium alloy members with each other or amagnesium alloy member and a heterogeneous material by means of a bolt,wherein a cationic electrodeposition coating having a film thickness of15 μm or more is provided at least on the surface of said magnesiumalloy member contacting with a bolt head, a powder coating having a filmthickness of 40 to 150 μm is provided on the surface of said cationicelectrodeposition coating, zinc-nickel plating and then cosmer treatmentare carried out on said bolts, and an alumite-treated aluminum washer isinterposed between said bolt head and said magnesium alloy member.
 2. Abolt tightening structure of a magnesium alloy member in accordance withclaim 1, wherein the thickness of an alumite layer on saidalumite-treated aluminum washer is 10 μm or more.
 3. A bolt tighteningstructure of a magnesium alloy member for tightening magnesium alloymembers with each other or a magnesium alloy member and a heterogeneousmaterial by means of a bolt, wherein a cationic electrodepositioncoating having a film thickness of 15 μm or more is provided at least onthe surface of said magnesium alloy member contacting with a bolt head,a powder coating having a film thickness of 40 to 150 μm is provided onthe surface of said cationic electrodeposition coating, zinc-nickelplating and then cosmer treatment are carried out on said bolts, and aniron or aluminum alloy washer coated by a cationic electrodepositioncoating having a film thickness of 15 μm or more is interposed betweensaid bolt head and said magnesium alloy member.
 4. A bolt tighteningstructure of a magnesium alloy member in accordance with claim 1,wherein chromate treatment is carried out on said bolt after saidzinc-nickel plating and before said cosmer treatment, and any one ofchromate treatment, chrome phosphate treatment, and manganese phosphatetreatment is carried out on said magnesium alloy member before providingsaid cationic electrodeposition coating.
 5. A bolt tightening structureof a magnesium alloy member in accordance with claim 2, wherein chromatetreatment is carried out on said bolt after said zinc-nickel plating andbefore said cosmer treatment, and any one of chromate treatment, chromephosphate treatment, and manganese phosphate treatment is carried out onsaid magnesium alloy member before providing said cationicelectrodeposition coating.
 6. A bolt tightening structure of a magnesiumalloy member in accordance with claim 3, wherein chromate treatment iscarried out on said bolt after said zinc-nickel plating and before saidcosmer treatment, and any one of chromate treatment, chrome phosphatetreatment, and manganese phosphate treatment is carried out on saidmagnesium alloy member before providing said cationic electrodepositioncoating.